This invention relates to an optical waveguide, an optical module, and an optical system using the same, and more particularly to, an optical waveguide, an optical module, and an optical system using the same which are suitable for optical circuit devices, such as optical star couplers, optical multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, etc., and optical transmission systems.
At present, many optical systems, such as an optical subscriber's system, an optical CATV, an optical submarine cable transmission system and an optical information processing system, have been actively developed. For the configuration of such optical systems, optical circuit devices, such as optical star couplers, optical multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, and optical transmission modules, in which such optical circuit devices such as a semiconductor laser and/or a photodiode, etc. are incorporated, are essential devices. An optical fiber-type device and an optical waveguide-type device are known to be used for such optical circuit devices. The optical waveguide-type device is expected to achieve a small size, low-cost and productive device because it also has the functions of the optical fiber-type device.
An optical waveguide using a semiconductor substrate like silicon and an optical waveguide using a silica glass substrate are already known. However, the optical waveguide using the silica glass substrate is more advantageous because it can be connected to an optical fiber by a fusing technique and less polarization-dependent loss is obtained.
A conventional optical waveguide comprises a silica glass substrate, at least one core waveguide formed thereon, and a cladding layer covering the core waveguide, wherein a certain amount of at least one dopant is added to both the core waveguide and the cladding layer so that the refractive index of the core waveguide is higher than that of the cladding layer. For fabricating the conventional optical waveguide, a silica glass substrate wafer is prepared, and a doped SiO.sub.2 glass layer is deposited by electron-beam deposition, which doped SiO.sub.2 glass layer is finally formed into a core waveguide. Next, a metal mask is formed on the doped SiO.sub.2 glass layer by sputtering, and a photoresist layer is formed on the metal mask by photolithography. After that, a core waveguide is patterned on the substrate by reactive-ion etching. At this step, the substrate is treated at a high temperature of more than 1200.degree. C. in order to stabilize the refractive index of the core waveguide. Next, a SiO.sub.2 porous glass layer as a cladding layer is formed by flame deposition by hydrolyzing source gases, then heated and consolidated at more than 1200.degree. C., and the cladding layer of transparent glass is obtained. Finally, the wafer is diced into a plurality of optical waveguides by a blade.
In the conventional optical waveguide and the conventional optical device using the same, however, there are disadvantages in that its connecting loss is likely to be extremely high, therefore the yield of fabrication thereof is low. Deformation of the substrate occurs by the high temperature treatment in the fabrication process, which causes a difference in the axes between the core waveguide and an optical fiber. Such deformations vary not only along the optical waveguide but also on the plane of the silica glass wafer where the optical waveguides are formed, and wafer by wafer.
Another disadvantage, is that the expected optical characteristics of optical devices, such as optical multiplexers/demultiplexer and wavelength-independent optical couplers are not obtained, because they depend on the lengths of the core waveguides. Furthermore, there is a disadvantage in that an absorption loss at a wavelength of 1.39 .mu.m exists, which seems to be caused by the existence of hydroxyl groups.